This invention relates to the field of can construction and more specifically deals with the construction of two-piece cans for use in the sealing of perishable foods. A trend has developed in the industry to utilize two-piece cans with their associated advantage of less joints in the manufacturing steps.
In the prior art three-piece cans were used primarily for the sealing of perishable foods. This type of construction results in a double seam around the interface of the top and bottom panels with the sidewall. This double seam extends outside of the sidewall. The prior art two-piece cans utilize a cup portion to connect with a top panel forming a double seam near the upper end. However, the cup portion in many instances has a smooth interface with the bottom panel, or has a bead significantly different from the double seam. Such a construction poses problems with respect to the present machinery utilized by canners in their canning operations designed originally for three-piece cans. One problem relates to the fact that the labeling machines do not incorporate apparatus to position the cans in any particular orientation with regard to a top or bottom. Since in three-piece cans the double seam at both ends of the sidewall are exactly the same, the can can be placed in either of two orientations for the placement of the label. It is necessary that the two double seams have essentially the same outside configuration with similar lip flanges, so that regardless of the can orientation when the label is attached to the can whichever double seam bead that is at the top of the label will accommodate a typical household can opener.
A further problem with prior art two-piece cans is that, when perishable foods are sealed within a can, the normal processing requires a recooking of the substance within the can after the can has been sealed. Consequently, it is necessary that the end panels be of such configuration and strength to withstand a certain amount of bulging as the interal pressure increases under the recooking. In the normal three-piece can construction the gauge of metal used for the end panels is greater than that used for the sidewalls. However, in two-piece can construction one of the end panels must be of the same gauge metal as the sidewall. Therefore, in the prior art two-piece cans the lighter gauge metal used in the sidewall, which is also the same as for one of the end panels, is not sufficient in strength to withstand the bulging and permanent buckling may result. This is attributable in large part to the fact that the normal three-piece can has end panels, incorporating standard profiles which are quite adequate for the heavier gauge metal to prevent buckling, but are inadequate for the lighter gauge metal used in two-piece cans.
When packaging three-piece cans in cartons for shipment, the double seams of respective adjacent cans mate with each other and provide protection to the labeling of the can during the shipment. In the prior art two-piece can construction where only one end of the can has a double seam, the packaging of the cans in cartons for shipment results in a loose orientation between the cans at one end, resulting in the cans hitting each other on the labels and damaging these labels.
A primary concern of canners is the ability to utilize their present machinery with two-piece cans as well as three-piece cans in order to allow them to phase in the use of two-piece cans. The prior art two-piece cans do not provide this flexibility, since their construction utilizes only one double seam and, therefore, the canning machinery would require some mechanism to orient the cans, so that the double seam for use with can openers will be adjacent the top of the label. However, canners in many instances would like to maintain the ability to interchange the use of three-piece and two-piece cans rather than orienting their machinery to a particular two-piece can configuration as shown by prior art two-piece cans. This interchangeability with normal three-piece cans has not been achieved with prior art two-piece cans, as prior methods of making two-piece cans have not resulted in a can having a bottom bead similar in configuration to the double seam.